The invention relates to a system for controlling the change of the read side for an optical reader for an information carrier or data medium and in particular a video disk.
It is known that information recorded on one or both sides of a transparent video disk can be read by means of a light beam, for example from a laser, which can be focused onto the recorded surface by appropriate optical means. Photoelectric cells positioned below the disk receive the light diffracted by the recording constituted, for example, by small holes, thus supplying an electrical signal modulated by the information recorded on the disk.
In the case of a video disk carrying information recorded on its two sides, in order to read side 2 (f.sub.2) it is merely necessary to transfer the focusing point to the face 2 (f.sub.2) by moving for example the optical focusing system in a direction perpendicular to the disk plane. Under these conditions, the light spot attains suitable dimensions to enable it to accurately read the small holes forming the recording on side 2 (f.sub.2), but is much too large to be influenced by the recording on side 1 (f.sub.1). In practice, it is merely necessary to displace the focusing point in an appropriate manner to immediately change from reading one of the sides of the disk to reading the other side without having to turn over the disk, as is the case in most double-sided disk systems (e.g. audio disks).
It is also known that the correct focusing of the light beam is usually obtained by means of a focus servocontrol having a feedback loop which maintains constant the distance between the optical device and the recorded surface of the disk. The connection and maintenance in place of the servodevice are obtained by the detection of the electrical signal corresponding to the recording.
The simplest way of changing focusing and therefore reading from one side to the other, consists of opening the focusing servodevice loop for a predetermined time T and imparting on the focusing device a translation movement corresponding to the displacement from one disk side to the other. This translation movement is obtained by applying to the focusing device a current pulse which is correctly calibrated in time and amplitude in such a way that the device is displaced precisely by the thickness of the disk for time T. At this time, it is merely necessary to close the focusing servodevice loop again for the latter to be connected to the opposite side. However, this very simple process does not give satisfactory results under practical conditions.
The translation movement from one side to the other involves two phases. During the first phase, the focusing device performs a movement which is uniformly accelerated throughout the current pulse time, whereas during the second phase the device performs a ballistic movement which is a function of the speed acquired during the first phase. However, this speed, as well as the travel during the first phase is dependent not only on the acceleration applied to the focusing device by means of the calibrated pulse, but also depends on the initial displacement speed of the device at the time of applying the current pulse. As it is a looped servosystem constantly correcting the focusing error, this initial speed is rarely zero and algebraically is added to the speed imparted to the focusing device, which has the effect of increasing or reducing the translation movement imparted to the focusing device during time T.
Moreover, through gravity acceleration, the translation movements of side 1 towards side 2 or from side 2 towards side 1 differ. Finally, other parameters can influence this translation movement, i.e. friction between the mechanical parts of the focusing device can vary as a function of temperature, humidity or other difficult to control parameters.
It is therefore difficult and relatively unreliable to pass from one side to the other by a process which consists of allocating a predetermined time to the change of side.